Fan blades of turbofan jet engines move high volumes of unheated, unexpanded atmospheric air circumferentially around a high pressure, heat expanded jet thrust that emanates centrally from a hot jet engine core. The air moved by the fan blades is termed “bypass” air, because such air is not mixed with fuel within the engine core to produce a primary engine thrust. The ratio of such bypass air to that of the primary engine thrust is called a bypass ratio. Those skilled in the art will appreciate advantages of utilizing bypass air, and that the fan blades are primarily designed for bypass ratio optimization with respect to any particular turbofan engine. Although such optimization is beyond the scope of this disclosure, it should be appreciated that optimization of an engine's particular bypass ratio is critical to minimizing fuel burn; i.e., the amount of fuel required for any given flight.
Fan blades constitute airfoils that are normally situated at the front end of a turbofan engine. As such, fan blades are subject to potential damage from deleterious impacts with foreign objects, including variable sized birds and geese, for example.
Approximately two-thirds of the outer portion of the leading edge of the fan blade is exposed to a so-called airfoil flow path. Since the fan blades is typically formed of an aluminum alloy base metal, the leading edge of the blade is covered with a hard metal sheath, most often via an adhesively applied layer of titanium metal. To minimize collision damage, the sheath normally directly covers only a structurally weaker, hence more vulnerable, outer portion of any given radial leading edge of the fan blade, typically because innermost radial portions of fan blades rotate at slower speeds and are thicker than their radially outer counterparts, hence less subject to collision damage.
Lightning strikes can pose a different type of hazard for aircraft structures. Damage risks due to such strikes must also be properly managed. Because the aluminum exterior of the fan blade is often coated to counter the effects of erosion, the aluminum body of the blade does not conduct electricity as well as the titanium sheath. Thus, providing a continuous path for lightning strikes has required the use of additional titanium strips and/or other structures to carry the energy of a lightning strike along a path of least resistance, e.g. the titanium, radially inwardly from the leading edge tip of the fan blade to a titanium rotor and/or titanium shaft, where it may be readily dissipated.
Finally, to the extent that the roots of the fan blades are most often secured to a rotor by means of dovetail portions adapted to extend axially within slots of the rotor, a system of reinforcing and/or strengthening turbojet fan blades is desirable, particularly from a standpoint of mitigating stresses and reducing potential metal fatigue issues in areas of associated fan blade air flow control platforms and/or in reduced thickness neck areas situated just radially outwardly of dovetail attachment root portions of the fan blade body.
In the related art, such structures have endured discontinuity issues and/or have had complex construction requirements. Thus, general fan blade strength improvements would be welcome in fan blade art for assuring simpler, yet more robust and dependable, structures along with improved lightning strike flow paths from fan blade tip to engine rotor.